1. Field of the Invention
The present invention relates to a device for acquiring and averaging samples of a noisy periodic signal.
Although the scope of the invention is not restricted to the video signal transmission field the noisy periodic signal can be particularly one of the test signals inserted into specific lines of the field blanking interval of a video signal. The characteristics of these insertion test signals and the parameters to be measured therefrom for determining the quality of a video signal transmission system are specified in the recommandations A 473-2 (1970) and A 569 (1978) published by the International Radiocommunication Consultative Committee (CCIR).
2. Description of the Prior Art
Mesuring equipments already known in prior art measure automatically the distorsions that the insertion test signals undergo, and evaluate the signal quality at various points along the video signal transmission medium. These measuring equipments can be grouped into two categories.
Those in the first category, existing in by for the largest numbers, marketed by Rhode and Schwarz, a GEC-Marconi Electrics Company and Philips, employ conventional signal acquiring and averaging devices which primarily consist of the following circuits. A plurality of conventional analog sampling and holding circuits equal to the number of samples to be acquired is inserted at the noisy signal input. Each of these sampling and holding circuits is organized around a switch which is closed throughout the sampling period on a capacitor intended for storing the sampled voltage in the form of a direct voltage. Then, analog circuits for filtering the sampled voltages make it possible to average their fluctuations in the presence of random noise. After this filtering, analog calculating units which are made up of operational amplifiers for example, calculate the various parameters to be measured based on the sample voltages and deliver the signals representative of the parameters in the form of direct voltages. A slow analog-to-digital converter connected to the calculating unit output sequentially converts, in digital form, the direct voltages delivered by the calculating units. According to the first category then, it turns out that the parameters are first of all acquired in analog form, thereby constituting the drawback of utilizing relatively costly analog calculating units. As a result, the number of samples is reduced.
The measuring equipment in the second category, more recent and marketed by Tektronic, Inc. makes wider use of digital techniques. The noisy signal to be analysed is sampled at a very high frequency, above the Nyquist frequency. Each sample is instantaneously converted into a digital form by means of a fast analog-to-digital converter. The digital values in relation to each sample, for several successive noisy signal periods, are stored into a memory. At the end of these successive periods, a microprocessor reads the stored digital values for each sample and calculates for these successive periods the average value of the sample, in the mathematical meaning of the term. The value of the sample is then very inaccurate if the signal is highly noisy during said successive periods.
Furthermore, it is a known fact that the current fast analog-to-digital converters have a relatively low resolution, at most 8 bits per sample, which limits the accuracy of the sampling and consequently the averaging in the second category measuring equipment.